The hardware business is dead and dusted. Amazon needs a platform to sell you things, not a difficult hardware business. Blackberry's only useful assets are their enterprise software and sales teams; everything else is noise.
Amazon is already in the hardware business and it is far from dead and dusted. Sorry to disagree. The Kindle Fire is a significant component in their business, it's currently running on Android, BB10 is an excellent OS + platform. Makes sense to buy BBRY, port to Kindle, disjoin from one of your biggest competitors and keep or eject the remaining pieces of BBRY hardware available on a case by case basis. The Q10 is a nice piece of hardware and BB10 is a great platform.
It's a natural fit and like I mentioned above Amazon was quite interested 18 months ago, I doubt much there has changed except the fact that the market cap for BBRY has fallen since then, making it an even more attractive asset to Amazon.